<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>一个表</title>

    <script>
        function StartTime() {
            var today = new Date();
            var h = today.getHours();
            var m=today.getMinutes();
            var s=today.getSeconds();// 在小于10的数字前加一个‘0’
            m=checkTime(m);
            s=checkTime(s);
            document.getElementById('txt').innerHTML = h + ":" + m + ":" + s;
            t = setTimeout(function () {
                StartTime(), 500
            });
        }

        function checkTime(i) {
            if(i < 10) {
                i = "0" + i;
            }
            return i;
        }
    </script>

</head>
<body onload="StartTime()">

<div id="txt"></div>

</body>
</html>